The present invention relates to a maximum likelihood sequence estimation apparatus which improves data transmission characteristic by eliminating distortion due to sampling timing error or inter symbol interferences (ISI) in case of transmitting data through time varying transmission paths causing inter symbol interferences.
Since it is difficult to obtain an optimum sampling timing from a receiving signal distorted by inter symbol interferences, a system is proposed to equalize and decode the signal using the signal sampled at a sampling frequency equal to an integer number of times of the symbol rate. One example of such system is to equalize and decode using a decision feedback equalizer of fractional interval. (See, for example, "Decision Feedback Equalization for Digital Cellular Radio" by S. Chennakeshu, et al., IEEE, Conf. Record on ICC '91, 339.4.1-339.4.5.)
A maximum likelihood sequence estimation apparatus is known to be an optimum system to eliminate distortion due to inter symbol interferences. (See, for example, "Digital Communications" by J. G. Proakis, McGraw-Hill, 1983.)
However, the conventional maximum likelihood sequence estimation system is incapable of correcting distortion due to sampling timing error because only known sampling frequency is equal to the symbol rate.